Configurable width buffered module having switch elements

ABSTRACT

A memory system architecture/interconnect topology includes a configurable width buffered memory module having a configurable width buffer device with at least one switch element. A buffer device, such as a configurable width buffer device, is positioned between or with at least one integrated circuit memory device positioned on a substrate surface of a memory module, such as a DIMM. A switch element is positioned on or off a memory module and includes two transistors in embodiments of the invention. One or more switch elements are coupled to one or more channels to allow for upgrades of memory modules in a memory system. An asymmetrical switch topology allows for increasing the number of memory modules to more than two memory modules without adding switch elements serially on each channel. Switch elements allow for increasing the number of ranks of memory modules in a system, while also achieving many of the benefits associated with point-to-point topology.

This application is a continuation-in-part of Ser. No. 10/766,131 filed on Jan. 28, 2004 (still pending); which is a continuation-in-part of U.S. patent application Ser. No. 10/272,024 filed on Oct. 15, 2002 (pending); which is a continuation of U.S. patent application Ser. No. 09/479,375 filed on Jan. 5, 2000 (now U.S. Pat. No. 6,502,161).

BACKGROUND OF THE INVENTION

This invention relates to memory systems, memory subsystems, memory modules or a system having memory devices. More specifically, this invention is directed toward memory system architectures that may include integrated circuit devices such as one or more controllers and a plurality of memory devices.

Some contemporary memory system architectures may demonstrate tradeoffs between cost, performance and the ability to upgrade, for example; the total memory capacity of the system. Memory capacity is commonly upgraded via memory modules or cards featuring a connector/socket interface. Often these memory modules are connected to a bus disposed on a backplane to utilize system resources efficiently. System resources include integrated circuit die area, package pins, signal line traces, connectors, backplane board area, just to name a few. In addition to upgradeability, many of these contemporary memory systems also require high throughput for bandwidth intensive applications, such as graphics.

With reference to FIG. 1, a representational block diagram of a conventional memory system employing memory modules is illustrated. Memory system 100 includes memory controller 110 and modules 120 a–120 c. Memory controller 110 is coupled to modules 120 a–120 c via control/address bus 130, data bus 140, and corresponding module control lines 150 a–150 c. Control/address bus 130 typically comprises a plurality of address lines and control signals (e.g., RAS, CAS and WE).

The address lines and control signals of control/address bus 130 are bussed and “shared” between each of modules 120 a–120 c to provide row/column addressing and read/write, precharge, refresh commands, etc., to memory devices on a selected one of modules 120 a–120 c. Individual module control lines 150 a–150 c are typically dedicated to a corresponding one of modules 120 a–120 c to select which of modules 120 a–120 c may utilize the control/address bus 130 and data bus 140 in a memory operation.

Here and in the detailed description to follow, “bus” denotes a plurality of signal lines, each having one or more connection points for “transceiving” (i.e., transmitting or receiving). Each connection point connects or couples to a transceiver (i.e., a transmitter-receiver) or one of a single transmitter or receiver circuit. A connection or coupling is provided electrically, optically, magnetically, by way of quantum entanglement or equivalently thereof.

With further reference to FIG. 1, memory system 100 may provide an upgrade path through the usage of modules 120 a–120 c. A socket and connector interface may be employed which allows each module to be removed and replaced by a memory module that is faster or includes a higher capacity. Memory system 100 may be configured with unpopulated sockets or less than a full capacity of modules (i.e., empty sockets/connectors) and provided for increased capacity at a later time with memory expansion modules. Since providing a separate group of signals (e.g., address lines and data lines) to each module is avoided using the bussed approach, system resources in memory system 100 are efficiently utilized.

U.S. Pat. No. 5,513,135 discloses a contemporary dual inline memory module (DIMM) having one or more discrete buffer devices.

Examples of contemporary memory systems employing buffered modules are illustrated in FIGS. 2A and 2B. FIG. 2A illustrates a memory system 200 based on a Rambus® channel architecture and FIG. 2B illustrates a memory system 210 based on a Synchronous Link architecture. Both of these systems feature memory modules having buffer devices 250 disposed along multiple transmit/receive connection points of bus 260.

In an upgradeable memory system, such as conventional memory system 100, different memory capacity configurations become possible. Each different memory capacity configuration may present different electrical characteristics to the control/address bus 130. For example, load capacitance along each signal line of the control/address bus 130 may change with two different module capacity configurations. However, using conventional signaling schemes, the bussed approaches lend efficiency towards resource utilization of a system and permits module interfacing for upgradeability.

There is a need for memory system architectures or interconnect topologies that provide flexible and cost effective upgrade capabilities while providing high bandwidth to keep pace with microprocessor operating frequencies.

BRIEF DESCRIPTION OF THE DRAWINGS

In the course of the detailed description to follow, reference will be made to the attached drawings, in which:

FIG. 1 illustrates a representational block diagram of a conventional memory system employing memory modules;

FIGS. 2A and 2B illustrate contemporary memory systems employing buffered modules;

FIGS. 3A and 3B illustrate a block diagram representing memory systems according to embodiments of the present invention;

FIG. 3C illustrates a block diagram representing a memory module that includes a configurable width buffer device according to an embodiment of the present invention;

FIGS. 4A, 4B, and 4C illustrate buffered memory modules according to embodiments of the present invention;

FIG. 5A illustrates a block diagram of a buffer device according to another embodiment of the present invention;

FIG. 5B illustrates a block diagram of a configurable width buffer device according to an embodiment of the present invention;

FIG. 5C illustrates a block diagram showing multiplexing and demultiplexing logic used in a configurable width interface of a buffer device shown in FIG. 5B according to an embodiment of the present invention;

FIG. 5D is a table showing control input states to achieve specified data widths in the configurable width buffer device shown in FIG. 5B;

FIGS. 5E and 5F illustrate configurable width buffered modules including a configurable width buffer device coupled to memory devices according to an embodiment of the present invention;

FIGS. 6A and 6B illustrate block diagrams of a memory system according to other embodiments of the present invention;

FIG. 7 illustrates a block diagram of a memory system employing a buffered quad-channel module according to an embodiment of the present invention;

FIG. 8A illustrates a block diagram of a large capacity memory system according to another embodiment of the present invention; and

FIGS. 8B and 8C illustrate another approach utilized to expand the memory capacity of a memory system in accordance to yet another embodiment of the present invention;

FIG. 9A illustrates a memory system including a buffered memory module having switch elements in accordance to an embodiment of the present invention;

FIG. 9B illustrates a switch element in accordance to an embodiment of the present invention;

FIG. 10A illustrates an address space for a single buffered memory module having switch elements in accordance to an embodiment of the present invention;

FIG. 10B illustrates an address space for two buffered memory modules having switch elements in accordance to an embodiment of the present invention;

FIGS. 11A–D illustrates a plurality of memory systems including one to four buffered memory modules having respective switch elements in accordance to embodiments of the present invention;

FIG. 12 illustrates a memory system including a buffered memory module having asymmetric switch elements in accordance to an embodiment of the invention;

FIG. 13A illustrates an address space for a single buffered memory module having asymmetric switch element in accordance to an embodiment of the present invention;

FIG. 13B illustrates an address space for two buffered memory modules having asymmetric switch elements in accordance to an embodiment of the present invention;

FIG. 14A illustrates an address space for a single buffered memory module having asymmetric switch elements in accordance to an embodiment of the present invention;

FIG. 14B illustrates an address space for two buffered memory modules having asymmetric switch elements when the two buffer memory modules do not have equal storage capacity in accordance to an embodiment of the present invention;

FIG. 15A illustrates managing an address space for a single buffered memory module having asymmetric switch elements in a memory system having buffered memory modules with unequal storage capacity in accordance to an embodiment of the present invention;

FIG. 15B illustrates managing an address space for two buffered memory modules having asymmetric switch elements when the two buffered memory modules have unequal storage capacity in accordance to an embodiment of the present invention;

FIG. 16A illustrates how a single buffered memory module is used in a four-socket memory system in accordance to an embodiment of the present invention;

FIG. 16B illustrates how two buffered memory modules are used in a four-socket memory system in accordance to an embodiment of the present invention;

FIG. 16C illustrates how three buffered memory modules are used in a four-socket memory system in accordance to an embodiment of the present invention;

FIG. 16D illustrates how four buffered memory modules are used in a four-socket memory system in accordance to an embodiment of the present invention;

FIGS. 17 and 19 illustrate different modes of operation of a buffered memory module in accessing storage cells in a memory device in accordance to an embodiment of the present invention;

FIGS. 18 and 20 illustrate different modes of operation of a buffered memory module in accessing respective storage cells in respective memory devices in accordance to an embodiment of the present invention;

FIGS. 21 and 23 illustrate different modes of operation of a buffered memory module in accessing storage cells in a memory device from signal lines in accordance to an embodiment of the present invention;

FIGS. 22 and 24 illustrate different modes of operation of a buffered memory module in accessing respective storage cells in respective memory devices from signal lines in accordance to an embodiment of the present invention.

DETAILED DESCRIPTION

The present invention relates to a memory system which includes one or more semiconductor memory devices coupled to a buffer device having a bypass circuit. The buffer device may be disposed on a memory module, housed in a common package along with memory devices, or situated on a motherboard, for example, main memory in a personal computer or server. The buffer device may also be employed in an embedded memory subsystem, for example such as one found on a computer graphics card, video game console or a printer.

In several embodiments, the buffer device having a bypass circuit provides for flexible system configurations, and several performance benefits. For example, a buffer device having a bypass circuit enables adding memory module upgrades to a memory system while reducing system delays for accessing information from the memory module upgrades. Further, the buffer device may be a configurable width buffer device to provide upgrade flexibility and/or provide high bandwidth among a variety of possible module configurations in the system. A plurality of buffer devices, configurable or otherwise, may be utilized in the memory system to provide high capacity, without compromising performance. A buffer device having configurable width functionality may be employed to allow memory subsystem bandwidth to scale as the system is upgraded or to allow theoretical maximum memory subsystem bandwidth to be achieved among possible memory module configurations. As specified herein, “configurable width” is used to denote that interfacing to the buffer device may be configured in a flexible manner, for example, by configuring how many parallel bits of data may be transferred with the buffer device.

In several embodiments, one or more busses or a plurality of point-to-point links may be used to couple one or more buffer devices to a master device (e.g., a controller or microprocessor device). A dynamic point-to-point link topology or any combination of point-to-point links or busses may be used to interface the master device and a corresponding buffer device.

In a specific embodiment, at least one point-to-point link connects at least one memory subsystem to the master, (e.g., a processor or controller). The memory system may be upgraded by coupling memory subsystems to the master via respective dedicated point-to-point links. Each memory subsystem includes a buffer device (e.g., a configurable width buffer device) that communicates to a plurality of memory devices. The master communicates with each buffer device via each point-to-point link. The buffer device may be disposed on a memory module along with the plurality of memory devices and connected to the point-to-point link via a connector. Alternatively, the buffer device may be disposed on a common printed circuit board or backplane link along with the corresponding point-to-point link and master.

“Memory devices” are a common class of integrated circuit devices that have a plurality of storage cells, collectively referred to as a memory array. A memory device stores data (which may be retrieved) associated with a particular address provided, for example, as part of a write or read command. Examples of types of memory devices include dynamic random access memory (DRAM), static random access memory (SRAM), and double data rate SDRAM (DDR). A memory device typically includes request decode and array access logic that, among other functions, decodes request and address information, and controls memory transfers between a memory array and routing path. A memory device includes a transceiver for transmitting and receiving data in an embodiment of the present invention. A transceiver includes a transmitter circuit to output data synchronously with respect to rising and falling edges of a clock signal as well as a receiver circuit in an embodiment of the present invention.

A “memory subsystem” is a plurality of memory devices, which may be interconnected with an integrated circuit device (e.g., a buffer device) providing access between the memory devices and an overall system, for example, a computer system. It should be noted that a memory system is distinct from a memory subsystem in that a memory system may include one or more memory subsystems. A “memory module” or simply just “module” denotes a substrate package housing or structure having a plurality of memory devices employed with a connector interface. For example, a memory module may be included in a single unitary package, as in a “system in package” (“SIP”) approach. In one type of SIP approach, the module may include a series of integrated circuit dies (i.e., memory devices and buffer device) stacked on top of one another and coupled via conductive interconnect. Solder balls or wire leads may be employed as the connector interface such that the module may be fixedly attached to a printed circuit board substrate. The connector interface may also be of a physically separable type that includes, for example, male and female portions such that the module is detachable from the rest of the system. Another SIP approach may include a number of memory devices and buffer device disposed, in a two dimensional arrangement, on a common substrate plane and situated inside a single package housing.

It follows from these definitions that a memory module having a buffer device (e.g., having a configurable width) isolating data, control, and address signals of the memory devices from the connector interface may be a memory subsystem. As referred to herein, the term “buffer device” may be interchangeable with “configurable width buffer device”, although this does not expressly imply that a “buffer device” must have a “configurable width” feature.

A connector interface as described herein, such as a memory module connector interface, is not limited to physically separable interfaces where a male connector or interface engages a female connector or interface. A connector interface also includes any type of physical interface or connection, such as an interface used in a SIP where leads, solder balls or connections from a memory module are soldered to a circuit board. For example, in the stacked die approach, a number of integrated circuit die (i.e., memory devices and buffer device) may be stacked on top of one another with a substrate forming the base and interface to a memory controller or processor via a ball grid array type of connector interface. As another example, the memory devices and buffer device may be interconnected via a flexible tape interconnect and interface to a memory controller via one of a ball grid array type connector interface or a physically separable socket type connector interface.

With reference to FIGS. 3A and 3B, block diagrams of a memory system according to embodiments of the present invention are illustrated. Memory systems 300 and 305 include a controller 310, a plurality of point-to-point links 320 a–320 n, and a plurality of memory subsystems 330 a–330 n. For simplicity, a more detailed embodiment of memory subsystem 330 a is illustrated as memory subsystem 340. Buffer device 350 and a plurality of memory devices 360 are disposed on memory subsystem 340. Buffer device 350 is coupled to the plurality of memory devices 360 via channels 370. Interface 375 disposed on controller 310 includes a plurality of memory subsystem ports 378 a–378 n. A “port” is a portion of an interface that serves a congruent I/O functionality. The memory subsystem ports 378 a–378 n may be included as a portion of a configurable width interface, for example as is described in some of the embodiments below.) One of memory subsystem ports 378 a–378 n includes I/Os, for sending and receiving data, addressing and control information over one of point-to-point links 320 a–320 n.

According to an embodiment of the present invention, at least one memory subsystem is connected to one memory subsystem port via one point-to-point link. The memory subsystem port is disposed on the memory controller interface, which includes a plurality of memory subsystem ports, each having a connection to a point-to-point link. In other embodiments, memory subsystems are connected to a memory subsystem port via a bus (i.e., a plurality of signal lines). A combination of bus and point-to-point connections may be used to connect the memory subsystem ports to each memory subsystem, for example, point-to-point links may be employed to transport data between the memory subsystem ports and each memory subsystem, and one or more busses may be used to transport control and/or addressing information between the memory subsystem ports and each memory subsystem.

A dynamic point-to-point topology may also be employed to connect the memory subsystem port to each of the memory subsystems. A dynamic point-to-point topology includes a first plurality of point-to-point connections between the memory controller and a first memory module and a second plurality of point-to-point connections between the memory controller and the second memory module in a first configuration. For example, when the second memory module is removed from the system and a second configuration is desired, the plurality of point-to-point connections are routed to the first memory module to retain system bandwidth between memory modules and the controller or increase the bandwidth to the first memory module. The routing may be performed in a number of ways including using a continuity module or switches. In an embodiment, a configurable width buffer device disposed on the first memory module provides the flexibility to configure the width of the first memory module to accept switch between the first and second configurations. That is the configurable width buffer device may provide the flexibility to configure the module to connect to the first plurality of point-to-point connections in the first configuration and to connect to the second plurality of point-to-point links in the second configuration.

In FIG. 3A, point-to-point links 320 a–320 n, memory subsystems 330 a–330 c (including mating connectors 380 a–n), and controller 310, are incorporated on a common substrate (not shown) such as a wafer or a printed circuit board (PCB) in memory system 300. In an alternate embodiment, memory subsystems are incorporated onto individual substrates (e.g., PCBS). The memory subsystems are then fixedly attached to a single substrate that incorporates point-to-point links 320 a–320 n and controller 310. In another alternate embodiment illustrated in FIG. 3B, memory subsystems 330 a–330 c are incorporated onto individual substrates that include connectors 390 a–390 c to support upgradeability in memory system 305. Corresponding mating connectors 380 a–380 n are connected to a connection point of each point-to-point link 320 a–320 n. Each of mating connectors 380 a–380 n interface with connectors 390 a–390 c to allow removal/inclusion of memory subsystems 330 a–330 c in memory system 305. In one embodiment, mating connectors 380 a–380 n are sockets and connectors 390 a–390 c are edge connectors disposed on an edge of each memory subsystems 330 a–330 c. Mating connectors 380 a–380 n, are attached to a common substrate shared with point-to-point links 320 a–320 n and controller 310.

With further reference to FIGS. 3A and 3B, buffer device 350 transceives and provides isolation between signals interfacing to controller 310 and signals interfacing to the plurality of memory devices 360. In a normal memory read operation, buffer device 350 receives control, and address information from controller 310 via point-to-point link 320 a and in response, transmits corresponding signals to one or more, or all of memory devices 360 via channels 370. One or more of memory devices 360 may respond by transmitting data to buffer device 350 which receives the data via one or more of channels 370 and in response, transmits corresponding signals to controller 310 via point-to-point link 320 a. Controller 310 receives the signals corresponding to the data at corresponding ports 378 a–378 n. In this embodiment, memory subsystems 330 a–330 n are buffered modules. By way of comparison, buffers disposed on the conventional DIMM module in U.S. Pat. No. 5,513,135 are employed to buffer or register control signals such as RAS, and CAS, etc., and address signals. Data I/Os of the memory devices disposed on the DIMM are connected directly to the DIMM connector (and ultimately to data lines on an external bus when the DIMM is employed in memory system 100).

Buffer device 350 provides a high degree of system flexibility. New generations of memory devices may be phased in with controller 310 or into memory system 300 by modifying buffer device 350. Backward compatibility with existing generations of memory devices (i.e., memory devices 360) may also be preserved. Similarly, new generations of controllers may be phased in which exploit features of new generations of memory devices while retaining backward compatibility with existing generations of memory devices.

Buffer device 350 effectively reduces the number of loading permutations on the corresponding point-to-point link to one, thus simplifying test procedures. For example, characterization of a point-to-point link may involve aspects such as transmitters and receivers at opposite ends, few to no impedance discontinuities, and relatively short interconnects. By way of contrast, characterization of control/address bus 130 (see FIG. 1) may involve aspects such as multiple transmit and receive points, long stub lines, and multiple load configurations, to name a few. Thus, the increased number of electrical permutations tends to add more complexity to the design, test, verification and validation of memory system 100.

With further reference to FIG. 3B an exemplary system that uses configurable width modules coupled in a dynamic point-to-point configuration may be described. In this exemplary embodiment, each of memory subsystems 330 a–330 c is a buffered memory module having a configurable width interface. In this embodiment, system capacity may scale without compromising performance when memory modules are added to the system to increase total memory capacity. For example, memory system may be populated with a single buffered memory module located, for example in mating connector 380 a (e.g., a socket connector), thus leaving point-to-point links 320 b–320 n coupled to unpopulated mating connectors 380 b–380 n. In this configuration, point-to-point links 320 b–320 n may be routed to access the single buffered memory module located in mating connector 380 a and routed using electrical or mechanical switches. The single buffered memory module located in mating connector 380 a is programmed to include an interface width that may accommodate the routed point-to-point links 320 b–320 n. U.S. patent application Ser. No. 09/797,099, (the Upgradeable Application”) entitled “Upgradeable Memory System with Reconfigurable Interconnect,” filed on Feb. 28, 2001, Attorney Docket No. RB1-017US which application is incorporated by reference herein and which application is assigned to the owner of the present application, describes a configurable memory module that is used in embodiments of the present invention to provide a dynamic point-to-point configuration. In particular, the configurable memory module taught by the Upgradeable Application is used with a configurable width buffer device 391, as described below, in embodiments of the present invention. In a point-to-point system, the minimum number of links per memory module is limited by the number of memory devices in a memory module that does not include a buffer device, but can be as low as one link for a memory module having a configurable width buffer device 391. Because memory modules having configurable width buffer devices allow for fewer links per memory module, more memory modules can be supported for a particular number of links from a master device.

FIG. 3C shows an example of a configurable width buffered module 395 that can be used in conjunction with the system described above. Configurable width buffered module 395 includes memory devices 360, channels 370, configurable width buffer device 391 and connector 390. Configurable width buffered module 395 is configurable or programmable such that information may be transferred using different numbers of interface connections 390 a in connector 390 provided by configurable width buffer device 391. In an embodiment of the present invention, interface connections 390 a includes a plurality of contacts, conducting elements or pins. In an embodiment illustrated by FIG. 3C, there are four possible configurations for configurable width buffer device 391. As used in the circuit described above, however, each module will be configured in one of two ways: (a) to use its full set of available interface connections 390 a, or (b) to use only a limited subset (half in the described example) of its interface connections 390 a.

In the following discussion, the modules' alternative configurations, and in particular the configurable width buffer device 391 alternate configurations, are referred to as having or using different “widths”. However, it should be noted that the capacities of the memory modules may or may not change with the different data widths, at least in an embodiment of the present invention. A module's full set of data stored on the associated memory devices is available regardless of the buffer interface width being used. With wider interface widths, different subsets of memory devices and memory cells may be accessed through different sets of interface connections. With narrower data or interface widths, the different subsets of memory devices and memory cells are accessed through a common set of interface connections. At such narrower interface widths, larger addressing ranges may be used to access data from one or more of the memory devices.

Configurable width buffered module 395 includes at least one memory device 360 a, of memory devices 360, that receive and transmit data bit signals through channels 370 (e.g., a plurality of signal lines). In the described embodiment, memory devices 360 are discretely packaged Synchronous type DRAM integrated circuits (ICs), for example, DDR memory devices, Direct Rambus® memory devices (DRDRAM), or “XDR™” memory devices, although the memory devices might be any of a number of other types, including but not limited to SRAM, FRAM (Ferroelectric RAM), MRAM (Magnetoresistive or Magnetic RAM), Flash, or ROM. singly or in combination.

In the embodiment illustrated in FIG. 3A, buffered modules added to upgrade memory system 300 (e.g., increase memory capacity) are accommodated by independent point-to-point links. Relative to a bussed approach, system level design, verification and validation considerations are reduced, due to the decreased amount of module inter-dependence provided by the independent point-to-point links. Additionally, the implementation, verification and validation of buffered modules may be performed with less reliance on system level environment factors.

Several embodiments of point-to-point links 320 a–320 n include a plurality of link architectures, signaling options, clocking options and interconnect types. Embodiments having different link architectures include simultaneous bi-directional links, time-multiplexed bi-directional links and multiple unidirectional links. Voltage or current mode signaling may be employed in any of these link architectures.

Clocking methods employed in the synchronization of events in point-to-point link or bussed topologies include any of globally synchronous clocking (i.e., where a single clock frequency source is distributed to various devices in the system); source synchronous clocking (i.e., where data is transported alongside the clock from the source to destination such that the clock and data become skew tolerant) and encoding the data and the clock together. In one embodiment, differential signaling is employed and is transported over differential pair lines. In alternate embodiments, one or more common voltage or current references are employed with respective one or more current/voltage mode level signaling. In yet other embodiments, multi-level signaling-where information is transferred using symbols formed from multiple signal (i.e., voltage/current) levels is employed.

Signaling over point-to-point links 320 a–320 n or alternatively, over bussed topologies, may incorporate different modulation methods such as non-return to zero (NRZ), multi-level pulse amplitude modulation (PAM), phase shift keying, delay or time modulation, quadrature amplitude modulation (QAM) and Trellis coding. Other signaling methods and apparatus may be employed in point-to-point links 320 a–320 n, for example, optical fiber based apparatus and methods.

The term “point-to-point link” denotes one or a plurality of signal lines, each signal line having only two transceiver connection points, each transceiver connection point coupled to transmitter circuit, receiver circuit or transceiver circuit. For example, a point-to-point link may include a transmitter coupled at or near one end and a receiver coupled at or near the other end. The point-to-point link may be synonymous and interchangeable with a point-to-point connection or a point-to-point coupling.

In keeping with the above description, the number of transceiver points along a signal line distinguishes between a point-to-point link and a bus. According to the above, the point-to-point link consists of two transceiver connection points while a bus consists of more than two transceiver points.

One or more terminators (e.g., a resistive element) may terminate each signal line in point-to-point links 320 a–320 n. In several embodiments of the present invention, the terminators are connected to the point-to-point link and situated on buffer device 350, on a memory module substrate and optionally on controller 310 at memory subsystem ports 378 a–378 n. The terminator(s) connect to a termination voltage, such as ground or a reference voltage. The terminator may be matched to the impedance of each transmission line in point-to-point links 320 a–320 n, to help reduce voltage reflections. Signal lines of bussed topologies may also benefit from terminating end points or connection points where devices, such as buffer devices connect to those signal lines.

In an embodiment of the present invention employing multi-level PAM signaling, the data rate may be increased without increasing either the system clock frequency or the number of signal lines by employing multiple voltage levels to encode unique sets of consecutive digital values or symbols. That is, each unique combination of consecutive digital symbols may be assigned to a unique voltage level, or pattern of voltage levels. For example, a 4-level PAM scheme may employ four distinct voltage ranges to distinguish between a pair of consecutive digital values or symbols such as 00, 01, 10 and 11. Here, each voltage range would correspond to one of the unique pairs of consecutive symbols.

With reference to FIGS. 4A, 4B and 4C, buffered memory modules according to embodiments of the present invention are shown. Modules 400 and 450 include buffer device 405 and a plurality of memory devices 410 a–410 h communicating over a pair of channels 415 a and 415 b. In these embodiments channel 415 a communicates to memory devices 410 a–410 d and channel 415 b communicates to memory devices 410 e–410 h.

In an embodiment, channels 415 a and 415 b consist of a plurality of signal lines in a relatively short multi-drop bus implementation. The plurality of signal lines may be controlled impedance transmission lines that are terminated using respective termination elements 420 a and 420 b. Channels 415 a and 415 b are relatively short (i.e., are coupled to relatively few memory devices relative to a conventional memory system, for example see FIGS. 2A and 2B) and connect to an I/O interface (not shown) of each memory device via a short stub. Signal lines of channels 415 a and 415 b include control lines (RQ), data lines (DQ) and clock lines (CFM, CTM). The varieties of interconnect topologies, interconnect types, clocking methods, signaling references, signaling methods, and signaling apparatus described above in reference to point-to-point links 320 a–320 n may equally apply to channels 415 a and 415 b.

In accordance with an embodiment of the present invention, control lines (RQ) transport control (e.g., read, write, precharge . . . ) information and address (e.g., row and column) information contained in packets. By bundling control and address information in packets, protocols required to communicate to memory devices 410 a–410 h are independent of the physical control/address interface implementation.

In alternate embodiments, control lines (RQ) may comprise individual control lines, for example, row address strobe, column address strobe, etc., and address lines. Individual point-to-point control and address lines increase the number of parallel signal connection paths, thereby increasing system layout resource requirements with respect to a narrow “packet protocol” approach. In one alternate embodiment illustrated in FIG. 6A, individual device select lines 633 a and 633 b are employed to perform device selection. Individual device select lines 633 a and 633 b decrease some latency consumed by decoding device identification that normally is utilized when multiple devices share the same channel and incorporate individual device identification values.

Clock lines of channels 415 a and 415 b include a terminated clock-to-master (CTM) (i.e., clock to buffer) and clock-from-master (CFM) (i.e., clock from buffer) line. In a source synchronous clocking method, CTM may be transition or edge aligned with control and/or data communicated to buffer device 405 from one or more of memory devices 410 a–410 d in, for example, a read operation. CFM may be aligned with or used to synchronize control and/or data from the buffer to memory in, for example, a write operation.

Although two channels 415 a and 415 b are shown in FIG. 4A, a single channel is also feasible. In other embodiments, more than two channels may be incorporated onto module 400. It is conceivable that if each channel and memory device interface is made narrow enough, then a dedicated channel between each memory device and the buffer device may be implemented on the module. The width of the channel refers to the number of parallel signal paths included in each channel. FIG. 4B illustrates a quad-channel module 450 having channels 415 a–415 d. In this embodiment, channels 415 c and 415 d are routed in parallel with channels 415 a and 415 b to support more memory devices (e.g., 32 memory devices). By incorporating more channels and additional memory devices, module 450 (FIG. 4B) may be implemented in memory systems that require large memory capacity, for example, in server or workstation class systems.

In alternate embodiments, channels 415 a and 415 b may operate simultaneously with channels 415 c and 415 d to realize greater bandwidth. By operating a plurality of channels in parallel, the bandwidth of the module may be increased independently of the memory capacity. The advantages of greater bandwidth may be realized in conjunction with larger capacity as more modules incorporated by the memory system 305 (see FIG. 3B) increase the system memory capacity. In other alternate embodiments, the modules are double sided and channels along with corresponding pluralities of memory devices are implemented on both sides. Using both sides of the module increases capacity or increases bandwidth without impacting module height. Both capacity and bandwidth may increase using this approach. Indeed, these techniques may increase capacity and bandwidth singly or in combination.

Other features may also be incorporated to enhance module 400 in high capacity memory systems, for example, additional memory devices and interface signals for error correction code storage and transport (ECC). Referring to FIG. 4C, memory devices 410 i and 410 r intended for ECC are disposed on module 470.

In one embodiment, memory devices 410 a–410 h are Rambus® Dynamic Random access Memory (RDRAM) devices operating at a data rate of 1066 Mbits/sec. Other memory devices may be implemented on module 400, for example, Double Data Rate 2 (DDR2) DRAM devices and Synchronous DRAM (SDRAM) devices. Utilizing buffer device 405 between the memory devices and controller in accordance with the present invention (e.g., see FIG. 3) may feasibly render the type of memory device transparent to the system. Different types of memory devices may be included on different modules within a memory system, by employing buffer device 405 to translate protocols employed by controller 310 to the protocol utilized in a particular memory device implementation.

With reference to FIG. 5A, a block diagram of a buffer device according to an embodiment of the present invention is illustrated. Buffer device 405 includes interface 510, interfaces 520 a and 520 b, multiplexers 530 a and 530 b, request & address logic 540, write buffer 550, optional cache 560, computation block 565, clock circuit 570 a–b and operations circuit 572.

In an embodiment, interface 510 couples to external point-to-point link 320 (e.g., point-to-point links 320 a–320 n in FIGS. 3A and 3B). Interface 510 includes a port having transceiver 575 (i.e. transmit and receive circuit) that connects to a point-to-point link. Point-to-point link 320 comprises one or a plurality of signal lines, each signal line having no more than two transceiver connection points. One of the two transceiver connection points is included on interface 510. Buffer device 405 may include additional ports to couple additional point-to-point links between buffer device 405 and other buffer devices on other memory modules. These additional ports may be employed to expand memory capacity as is described in more detail below. Buffer device 405 may function as a transceiver between point-to-point link 320 and other point-to-point links. FIGS. 8B and 8C illustrate some buffer-to-buffer connection topology embodiments, while one of ordinary skill in the art would appreciate that there many more embodiments.

In one embodiment, termination 580 is disposed on buffer device 405 and is connected to transceiver 575 and point-to-point link 320. In this embodiment, transceiver 575 includes an output driver and a receiver. Termination 580 may dissipate signal energy reflected (i.e., a voltage reflection) from transceiver 575. Termination 580 may be a resistor or capacitor or inductor, singly or a series/parallel combination thereof. In alternate embodiments, termination 580 may be external to buffer device 405. For example, termination 580 may be disposed on a module substrate or on a memory system substrate.

In another approach, signal energy reflected from transceiver 575 may be utilized in a constructive manner according to an embodiment. By correctly placing a receive point spaced by a distance from the end of point-to-point link 320, a reflected waveform is summed with an incident waveform to achieve a greater signal amplitude. In this approach, layout space may be saved by eliminating termination 580. System power may also be saved using this approach since smaller incident voltage amplitude waveforms may be employed. This approach may be equally applicable to the transceiver end of the point-to-point link, or to channels 415 a and 415 b (see FIGS. 4A to 4C).

With further reference to FIG. 5A, interfaces 520 a and 520 b receive and transmit to memory devices disposed on the module (e.g., see FIGS. 4A, 4B and 4C) via channels. Ports included on interfaces 520 a and 520 b connect to each channel. In alternate embodiments of the present invention, interfaces 520 a and 520 b include any number of channels e.g., two, four, eight or more channels.

According to an embodiment of the present invention, multiplexers 530 a and 530 b perform bandwidth-concentrating operations, between interface 510 and interfaces 520 a and 520 b, as well as route data from an appropriate source (i.e. target a subset of channels, internal data, cache or write buffer). The concept of bandwidth concentration involves combining the (smaller) bandwidth of each channel in a multiple channel embodiment to match the (higher) overall bandwidth utilized in a smaller group of channels. This approach typically utilizes multiplexing and demultiplexing of throughput between the multiple channels and smaller group of channels. In an embodiment, buffer device 405 utilizes the combined bandwidth of interfaces 520 a and 520 b to match the bandwidth of interface 510. Bandwidth concentration is described in more detail below.

Cache 560 is one performance enhancing feature that may be incorporated onto buffer device 405. Employing a cache 560 may improve memory access time by providing storage of most frequently referenced data and associated tag addresses with lower access latency characteristics than those of the memory devices. Computation block 565 may include a processor or controller unit, a compression/decompression engine, etc, to further enhance the performance and/or functionality of the buffer device. In an embodiment, write buffer 550 may improve interfacing efficiency by utilizing available data transport windows over point-to-point link 320 to receive write data and optional address/mask information. Once received, this information is temporarily stored in write buffer 550 until it is ready to be transferred to at least one memory device over interfaces 520 a and 520 b.

A serial interface 574 may be employed to couple signals utilized in initialization of module or memory device identification values, test function, set/reset, access latency values, vendor specific functions or calibration. Operations circuit 572 may include registers or a read-only memory (ROM) to store special information (e.g., vendor, memory device parameter or configuration information) that may be used by the controller. Operations circuit may reduce costs by eliminating the need for separate devices on the module conventionally provided to perform these features (e.g., serial presence detect (SPD) employed in some conventional DIMM modules). An SPD device is a non-volatile memory device included on a memory module. The SPD stores information used by the remainder system to properly configure the memory devices upon boot of the system.

According to an embodiment of the present invention, sideband signals are employed to handle special functions such as reset, initialization and power management functions. In addition, sideband signals may be employed to configure the width of the buffer device. Sideband signals are connected via serial interface 574 and are independent from point-to-point link 320 for handling the special functions. In other embodiments sideband signals are independently coupled to memory devices 410 a–410 h to directly promote initialization, reset, power-up or other functionality independently of buffer device 405. Other interconnect topologies of sideband signals are possible. For example, sideband signals may be daisy chained between buffer devices and coupled to the memory controller or daisy chained between all memory devices to the memory controller. Alternatively, dedicated sideband signals may be employed throughout.

Clock circuit 570 a–b may include clock generator circuit (e.g., Direct Rambus® Clock Generator), which may be incorporated onto buffer device 405 and thus may eliminate the need for a separate clock generating device. Here, module or system costs may be decreased since the need for a unique clock generator device on the module or in the system may be eliminated. Since reliability to provide adequate clocking on an external device is eliminated, complexity is reduced since the clock may be generated on the buffer device 405. By way of comparison, some of the conventional DIMM modules require a phase lock loop (PLL) generator device to generate phase aligned clock signals for each memory device disposed on the module. In an embodiment of the present invention, clock circuit 570 a–b is used to resynchronize information transferred by bypass circuits described below.

According to an embodiment of the present invention, clock circuit 570 a–b includes one or more clock alignment circuits for phase or delay adjusting internal clock signals with respect to an external clock (not shown). Clock alignment circuit may utilize an external clock from an existing clock generator, or an internal clock generator to provide an internal clock, to generate internal synchronizing clock signals having a predetermined temporal relationship.

FIG. 5B illustrates a configurable width buffer device 391 as seen in FIG. 3C in an embodiment of the present invention. Configurable width buffer device 391 includes like numbered components as shown in FIG. 5A and, in an embodiment, may operate as described above. Configurable width buffer device 391 also includes configurable width interface 590, state storage 576, configurable serialization circuit 591 and interface 596. In an embodiment, a plurality of contacts, solder balls or pins are included to provide electrical connections between interface 596 and connector 390 (FIG. 3C) via signal line traces routed on or through out a substrate portion of the module.

Also, in an embodiment of the present invention, one or more transceiver 575 (FIG. 5B) and termination 580 are associated with each port in interface 596. In this specific embodiment, transceiver 575 includes a transmit circuit to transmit data onto a signal line (external to configurable width buffer device 391) and a receiver circuit to receive a data signal on the same signal line. In an alternate embodiment, the transmit circuit is multiplexed with the data received by the receiver circuit. In still a further embodiment of the present invention, the transmit circuit transmits data and the receiver circuit receives data simultaneously.

In another embodiment, the transceiver 575 includes separate unidirectional transmit and receive circuits, each having dedicated resources for communicating data on and off configurable width buffer device 391. In this embodiment, unidirectional transmitter circuit transmits data onto a first signal line disposed on (external to configurable width buffer device 391, for example, disposed on configurable width buffered module 395). In addition, unidirectional receiver circuit receives data from a second signal line.

In another embodiment of the present invention, a transmit circuit transmits a differential signal that includes encoded clock information and a receiver circuit receives a differential signal that includes encoded clock information. In this embodiment, clock and data recovery circuit is included to extract the clock information encoded with the data received by the receiver circuit. Furthermore, clock information is encoded with data transmitted by the transmit circuit. For example, clock information may be encoded onto a data signal, by ensuring that a minimum number of signal transitions occur in a given number of data bits.

In an embodiment of the present invention, any multiplexed combination of control, address information and data intended for memory devices coupled to configurable width buffer device 391 is received via configurable width interface 590, which may, for example extract the address and control information from the data. For example, control information and address information may be decoded and separated from multiplexed data and provided on lines 595 to request & address logic 540 from interface 596. The data may then be provided to configurable serialization circuit 591. Request & address logic 540 generates one or more output enable (OE) signals to enable bypass circuits as described below.

Interfaces 520 a and 520 b include programmable features in embodiments of the present invention. A number of control lines between configurable width buffer device 391 and memory devices are programmable in order to accommodate different numbers of memory devices on a configurable width buffered module 395 in an embodiment of the present invention. Thus, more dedicated control lines are available with increased memory device memory module configuration. Using programmable dedicated control lines avoids any possible load issues that may occur when using a bus to transfer control signals between memory devices and a configurable width buffer device 391. In another embodiment of the present invention, an additional complement data strobe signal for each byte of each memory device may be programmed at interfaces 520 a and 520 b to accommodate different types of memory devices on a configurable width buffered memory module 395, such as legacy memory devices that require such a signal. In still a further embodiment of the present invention, interfaces 520 a and 520 b are programmable to access different memory device widths. For example, interfaces 520 a and 520 b may be programmed to connect to 16 “x4” width memory devices, 8“x8” width memory devices or 4“x16” width memory devices.

Configurable width buffer device 391 has a maximum buffer device interface width equivalent to the number of data pins or contacts provided on the buffer device's package or interface 596. In an embodiment of the present invention, interface 596 includes 128 pins of which selectable subsets of 1, 2, 4, 8, 16, 32, 64 or all 128 pins (W_(DP)) may be used in order to configure the width of configurable width buffer device 391. Configurable width buffer device 391 also has a maximum memory device access width defined as the largest number of bits that can be accessed in a single memory device transfer operation to or from configurable width buffer device 391. Using the techniques described herein, the configurable width buffer device 391 may be programmed or configured to operate at interface widths and memory device access widths other than these maximum values.

In an embodiment illustrated by FIG. 5B, a serialization ratio is defined as follows: RS=W_(A):W_(DP)

Where:

-   -   R_(S)=Serialization Ratio     -   W_(A)=Memory Device Access Width     -   W_(DP)=Configured Buffer Device Interface Width

For example, if the memory device access width W_(A) is 128-bits and the configurable width buffer device interface width W_(DP) is 16-bits, the serialization ratio is 8:1. For the described embodiment, the serialization ratio remains constant for all configured buffer device interface widths, so that the memory device access width scales proportionally with configured buffer device interface width. In other embodiments, the serialization ratio could vary as the configured buffer device interface width varies. In other embodiments, the serialization ratio may change and the memory device access width may remain constant.

Configurable serialization circuit 591 performs serialization and deserialization functions depending upon the desired serialization ratio as defined above. As the memory device access width is reduced from its maximum value, memory device access granularity (measured in quanta of data) is commensurately reduced, and an access interleaving or multiplexing scheme may be employed to ensure that all storage locations within memory devices 360 can be accessed. The number of signal lines of channels 370 may be increased or decreased as the memory device access width changes. Channels 370 (FIG. 3C) may be subdivided into several addressable subsets. The address of the transaction will determine which target subset of channels 370 will be utilized for the data transfer portion of the transaction. In addition, the number of transceiver circuits included in interface 520 a and 520 b that are employed to communicate with one or more memory devices may be configured based on the desired serialization ratio. Typically, configuration of the transceivers may be effectuated by enabling or disabling how many transceivers are active in a given transfer between one or more memory devices and configurable width buffer device 391.

In an embodiment of the present invention, a serialization ratio SR between a primary channel and secondary channels is defined below. In an embodiment of the present invention, communications between primary (e.g. link or bus) and secondary channels (e.g. channel 370 having a plurality of signal lines) have matched bandwidth (i.e. no compression, coding/ECC overhead, or caching). SR=BW_(P):BW_(S)

where:

-   -   BW_(P)=bandwidth of primary channel (bits/sec/signal line)     -   BW_(S)=bandwidth of secondary channel (bits/sec/signal line)

The minimum number of secondary channel signal lines required to participate per transaction between primary and secondary channels is: [W_(DPT,S) =W _(DP,P) *SR

where:

-   -   W_(DP,P)=programmed data path width for primary channel     -   W_(DP,S)=total data path width for secondary channel     -   W_(DPT,S)=minimum number of secondary channel signal lines         required to participate in each transaction

If the total number of secondary channel signal lines per configurable width buffer device 391, W_(DP,S) is greater than the minimum number required per transaction, W_(DPT,S), then configurable width buffer device 391 may employ a configurable datapath router within interface 590 to route requests between the primary channel and the target subset of secondary channel signal lines for each transaction. According to a preferred embodiment, the target subset of secondary channel signal lines may be selected via address bits provided as part of the primary channel request. By accessing a subset of secondary channel signal lines per transaction, a number of benefits may be derived. One of these benefits is reduced power consumption. Another benefit is higher performance by grouping memory devices into multiple independent target subsets (i.e. more independent banks).

Operations circuit 572 (similarly, as described above) is included in configurable width buffer device 391 in an embodiment of the present invention. Operations circuit 572 includes storage circuit to store information used by the system in which the configurable width buffer device 391 is situated in, for example, to perform configuration operations. Alternatively the information may be stored in a serial presence detect device (SPD). Alternatively, the information may be stored in a number of different physical locations, for example, in a register within a memory controller or a separate integrated circuit on a system motherboard. Operations circuit 572 may store information representing a possible number of configurations of configurable width buffer device 391 and/or configurable width buffered module 395 in embodiments of the present invention. Other information, which may be stored includes, but is not limited to memory device parameters, such as access times, precharge times, setup and hold times, allowable skew times, etc. The functionality of operations circuit 572 may be included on an SPD device for example, an EEPROM device, that is readable by the system, such as a controller, to determine the capabilities of a configurable width buffer device 391 and/or configurable width buffered module 395. A controller can then initialize or set system parameters, such as a data path or interface width, based on the values stored in the SPD device by generating control signals to a control terminal of configurable width buffered module 395. For example, the SPD device may indicate to a controller that configurable width buffer device 391 has a maximum width of 64 as opposed to a maximum width of 128. In an alternate embodiment of the present invention, the SPD device stores a number of serialization ratios, for example, which may be programmed into a register, located on configurable width buffer device 391.

State storage 576 may store a value that is repeatedly programmable or changeable to indicate different buffer device interface widths. The value stored in state storage 576 may be decoded to establish the desired configuration of configurable width interface 590. In addition, state storage may store a plurality of values, for example, a first value that represents the desired width of configurable width interface 590, and a second value that represents the desired width of one or more of interfaces 520 a and 520 b.

State storage may be programmed upon initialization by a controller device that communicates, to the configurable width buffer device, a value, which corresponds to the width of controller. The configurable width buffer device 391 may also automatically detect what the width of the interface of the controller device is upon power-up and set it's own value in state storage 576 accordingly.

In an embodiment of the present invention as illustrated by FIG. 5B, state storage 576 is a programmable register comprising two programmable memory cells, latches, or other mechanisms for storing state information. Within the two cells, two bits are stored. The two bits can represent four different values, through different combinations of bit values (Ex: 00=x1, 01=x2, 10=x4, 11=x8). The different stored values correspond to different programmed buffer device widths. In an embodiment of the present invention, state storage 576 outputs to request & address logic 540 and configurable width interface 590. In FIG. 5B, a state storage 576 is fabricated within each of configurable width buffer device 391. However, state storage 576 can alternatively be located in a number of different physical locations. For example, stage storage 576 might be a register within a memory controller or on a system motherboard. If the storage register is external to configurable width buffer device 391, the width selection information is communicated to configurable width buffer device 391 via electrical signals propagated through module connector 390. In an alternate embodiment of the present invention, a controller transfers width selection information by way of control signals to a control terminal of configurable width buffer device 391.

Various types of state storage are possible. In the described embodiment, the state storage takes the form of a width selection register or latch. This type of state can be easily changed via software during system operation, allowing a high degree of flexibility, and making configuration operations that are transparent to the end user. However, other types of state storage are possible, including but not limited to manual jumper or switch settings and module presence detection or type detection mechanisms. The latter class of mechanisms may employ pull-up or pull-down resistor networks tied to a particular logic level (high or low), which may change state when a module is added or removed from the system.

State storage 576 can be repeatedly programmed and changed during operation of configurable width buffer device 391 to indicate different interface widths. Changing the value or values of state storage 576 changes the interface width of configurable width buffer device 391; even after configurable width buffer device 391 has already been used for a particular width. In general, there is no need to power-down or reset configurable width buffer device 391 when switching between different interface widths, although this may be required due to other factors.

There are many possible ways to implement a state storage 576. Commonly, a register is defined as a state storage 576 that receives a data input and one or more control inputs. The control inputs determine when the storage node within the register will sample the data input. Some time after the register has sampled the input data, which data will appear on the output of the register.

The term register may apply either to a single-bit-wide register or multi-bit-wide register. In general, the number of bits in the width selection register is a function of the number of possible widths supported by the memory device, although there are many possible ways to encode this information.

FIG. 5C illustrates another embodiment of configurable width interface 590 shown in FIG. 5B. FIG. 5C illustrates a multiplexer/demultiplexer circuit 597, for example that may be disposed in configurable serialization circuit 591 (FIG. 5B) to perform multiplexing/demultiplexing functions. For the embodiment illustrated by FIG. 5C, the serialization ratio is 1:1. Serialization ratios greater than 1:1 are possible with the addition of serial-to-parallel (e.g., during write operations, for data intended to be written to the memory devices) and parallel-to-serial (e.g., during read operations, for data read from the memory device to be provided to the controller device) conversion circuits.

Multiplexer/demultiplexer circuit 597 includes four pairs of read and write data line pairs 594 a–d coupled, by way of configurable width buffer device 391, to respective memory devices 360. In an alternate embodiment of the present invention, data line pairs 594 a–d are coupled to a memory array in a memory device 360 a by way of configurable width buffer device 391.

Generally, multiplexer/demultiplexer circuit 597 contains multiplexing logic and demultiplexing logic. The multiplexing logic is used during read operations, and the demultiplexing logic is used during write operations. The multiplexing logic and demultiplexing logic are designed to allow one, two, or four (0–3) buffer device connections or pins in interface 596 to be routed to memory devices, and in particular individual memory cells.

In the one-bit wide configuration, buffer device data connection 0 can be routed to/from any of the four data line pairs 594 a–d, which may be coupled to respective memory devices or memory cells in a memory device. In the 2-bit wide configuration, buffer device data connections 0 and 1 can be routed to/from data line pairs 594 a–b or 594 c–d, respectively. In the 4-bit wide configuration, buffer device data connections 0, 1, 2, and 3 route straight through to/from data line pairs 594 a–d, respectively.

Likewise, further data paths may be constructed to couple greater than four configurable width buffer device 391 data connections in an embodiment of the present invention.

Multiplexer/demultiplexer circuit 597 includes input and output latches 597 f–m, two for each configurable width buffer device data connection in interface 596. Multiplexer/demultiplexer circuit 597 also comprises five multiplexers 597 a–e. Multiplexers 597 a–d are two-input multiplexers controlled by a single control input. Multiplexer 597 e is a four input multiplexer controlled by two control inputs.

Multiplexer/demultiplexer circuit 597 is configured to use two write control signals W_(A) and W_(B), and two read control signals R_(A) and R_(B). These signals control multiplexers 597 a–e. They are based on the selected data path width and bits of the requested memory address or transfer phase (see FIG. 5D, described below). State storage 576 (FIG. 5B) produces these signals in response to the programmed data width, whether the operation is a read or write operation, and appropriate addressing information.

FIG. 5D shows the control values used for data path widths of one, two, and four. FIG. 5D also indicates which of interface 596 connections are used for each data width.

When a width of one is selected during a read operation, the circuit allows data from any one of the four data line pairs 594 a–d (in particular, the read line) to be presented at interface 596 connection 0. Control inputs R_(A) and R_(B) determine which of data bit signals will be presented at any given time. R_(A) and R_(B) are set (at this data width) to equal the least significant two bits (A₁, A₀) of the memory address corresponding to the current read operation.

When a width of one is selected during a write operation, the circuit accepts the data bit signal from interface 596 data connection 0 and routes it to all of the four data line pairs 594 a–d (in particular, the write lines), simultaneously. Control inputs W_(A) and W_(B) are both set to a logical value of one to produce this routing.

When a width of two is selected during a read operation, the circuit allows any two of the four data bit signals associated with data line pairs 594 a–d (in particular, the read lines) to be present at interface 596 connections 0 and 1. To obtain this result, R_(A) is set to 0, and R_(B) is equal to the lower bit (A₀) of the memory address corresponding to the current read operation. R_(B) determines which of two pairs of data bit signals (594 a and 594 b or 594 c and 594 d) are presented at interface 596 connections 0 and 1 during any given read operation.

When a width of two is selected during a write operation, the circuit accepts the data bit signals from interface 596 connections 0 and 1, and routes them either to data line pairs 594 a and 594 b (in particular, write lines), or to data line pairs 594 c and 594 d (in particular, write lines). W_(A) and W_(B) are set to 0 and 1, respectively, to obtain this result.

When a width of four is selected by setting all of the control inputs (R_(A), R_(B), W_(A), and W_(B)) to 0, read and write data signals are passed directly between data line pairs 594 a–d and corresponding interface 596 data connections 3-0.

The circuit of FIG. 5C is just one example out of many possible embodiments of the present invention. At the expense of increased logic and wiring complexity, an embodiment of the present invention uses a more elaborate crossbar-type scheme that could potentially route any single data bit signal to any data pair line or to any of the interface 596 data connections. In still further embodiments of the present invention, the number and width of memory devices, number of buffer device data connections per buffer device, serialization ratios, and width of data paths may be varied, singly or in combination, from the exemplary numbers and widths provided in describing particular embodiments of the present invention.

In embodiments of the present invention, a master device, such as memory controller 110, includes configurable width interface 590, as described herein, to access at least one configurable width buffered module.

In embodiments of the present invention, interfaces 520 a and 520 b include multiplexer/demultiplexer circuit 597, also known as a type of partial crossbar circuit, for transferring signals between configurable width buffer device 391 and memory devices on configurable width buffered module 395. In an alternate embodiment of the present invention, a full crossbar circuit is used.

FIG. 5E illustrates a configurable width buffered module 650 including a configurable width buffer device 651 coupled to memory devices 652 and 653 in an embodiment of the present invention. Memory devices 652 and 653 include memory cells 652 a–b and 653 a–b, respectively.

Channels DQ1 and DQ2 are coupled to configurable width buffered module 650 at a connector interface that includes at least a first contact and a second contact, and in particular to configurable width buffer device 651. Channels DQ1 and DQ2 include a plurality of signal lines for providing signals to and from configurable width buffered module 650. In an alternate embodiment of the present invention, a single or more channels are coupled to configurable width buffered module 650. In an embodiment of the present invention, channels DQ1 and DQ2 are coupled to a master device, such as a controller.

Channels DQ3 and DQ4 are also coupled to configurable width buffer device 651 and are positioned in configurable width buffered module 650 in an embodiment of the present invention. Channels DQ3 and DQ4 couple configurable width buffer device 651 to memory devices 652 and 653. Channels DQ3 and DQ4 include a plurality of signal lines for providing signals to and from memory devices 652 and 653, and in particular memory cells 652 a–b and 653 a–b. In an alternate embodiment of the present invention, a single or more channels are coupled to memory devices 652 and 653. In alternate embodiments of the present invention, more or less memory devices are included in configurable width buffered module 650.

Configurable width buffer device 651 includes a plurality of transceiver circuits 651 a–f capable of transmitting and receiving signals on channels DQ1–4. Each transceiver circuit 651 a–f includes a transmitter circuit and a receiver circuit in an embodiment of the present invention. Transceiver 651 a is coupled to channel DQ1 and provides signals on channel DQ3. Transceiver 651 b is coupled to channel DQ3 and provides signals on channel DQ1. Transceiver 651 c is coupled to channel DQ1 and provides signals on channel DQ4. Transceiver 651 d is coupled to channel DQ4 and provides signals on channel DQ1. Transceiver 651 e is coupled to channel DQ2 and provides signals on channel DQ4. Transceiver 651 f is coupled to channel DQ4 and provides signals on channel DQ2. Both memory cells 652 a and 652 b are not accessed via channel DQ1 during a single access operation in an embodiment of the present invention.

In an embodiment of the present invention, configurable width buffered module 650 operates in at least two modes of operation. In a first mode of operation, memory cell 652 a and memory cell 652 b in memory device 652 are accessible from a first contact coupled to channel DQ1. In particular, signals are transferred between channel DQ1 and memory cell 652 a by using transceiver 651 a, transceiver 651 b and channel DQ3. Transceiver 651 a is used to write data signals to memory cell 652 a and transceiver 651 b is used to read data signals from memory cell 652 a. Signals are transferred between channel DQ1 and memory cell 652 b using transceiver 651 c, transceiver 651 d and channel DQ4. Transceiver 651 c is used to write data signals to memory cell 652 b and transceiver 651 d is used to read data signals from memory cell 652 b.

In a second mode of operation, memory cell 652 a and memory cell 652 b in memory device 652 are accessible from a first contact coupled to channel DQ1 and a second contact coupled to channel DQ2, respectively. In particular, signals are transferred between channel DQ1 and memory cell 652 a by using transceiver 651 a, transceiver 651 b and channel DQ3. Transceiver 651 a is used to write data signals to memory cell 652 a and transceiver 651 b is used to read data signals from memory cell 652 a. Signals are transferred between channel DQ2 and memory cell 652 b using transceiver 651 e, transceiver 651 f and channel DQ4. Transceiver 651 e is used to write data signals to memory cell 652 b and transceiver 651 f is used to read data signals from memory cell 652 b. In another embodiment of the present invention, memory device 653 is also coupled to channels DQ3 and DQ4 and includes memory cells 653 a and 653 b that are likewise accessible in at the two modes of operation described above.

FIG. 5F illustrates a configurable width buffered module 660 where channel DQ3 is coupled to memory cell 652 a in memory device 652 and channel DQ4 is coupled to memory cell 653 b in memory device 653. Like referenced components shown in FIG. 5F operate and are described above in regard to FIG. 5E. In this embodiment of the present invention, Configurable width buffer device 651 accesses two memory devices that do not share channels DQ3 and DQ4. There are two modes of operation in an embodiment of the present invention. In a first mode of operation, memory cell 652 a is accessed via channel DQ1 and memory cell 653 b is accessed via channel DQ2. In a second mode of operation, memory cell 652 a is accessed via channel DQ1 and memory cell 653 b is accessed via channel DQ1. Both memory cells 652 a and 653 b are not accessed via channel DQ1 during a single access operation in an embodiment of the present invention. FIG. 5F conceptually illustrates accessing memory cells by a configurable width buffer device 651. In order to clearly describe the present invention, one of ordinary skill in the art would appreciate that may components used in a buffer device and memory devices are not shown, such as memory device access logic and transceivers as well as configurable width buffer device 651 logic.

According to embodiments of the present invention, subsets of available memory devices on configurable width buffered module 395 are activated or powered-on during various modes of operation. Thus, configurable width buffered module 395 is able to achieve power savings by only powering particular memory devices.

With reference to FIGS. 6A, and 6B, block diagrams of a memory system according to embodiments of the present invention are illustrated. Memory system 600 includes modules 400 a and 400 b, controller 610, and populated primary point-to-point links 620 a and 620 b. Unpopulated primary point-to-point links 630 are populated by coupling additional modules (not shown) thereto. The additional modules may be provided to upgrade memory system 600. Connectors may be disposed at an end of each primary point-to-point link to allow insertion or removal of the additional modules. Modules 400 a and 400 b may also be provided with a connector or may be fixedly disposed (i.e., soldered) in memory system 600. Although only two populated primary point-to-point links are shown in FIG. 6A, any number of primary point-to-point links may be disposed in memory system 600, for example, three primary point-to-point links 400 a–400 c, as shown in FIG. 6B.

With reference to FIG. 7 and FIG. 4B, a block diagram of a memory system employing a buffered quad-channel module according to an embodiment of the present invention is illustrated. Memory systems 700 incorporate quad-channel modules 450 a–450 d, each coupled via point-to-point links 620 a–620 d respectively.

Referring to FIG. 4B, buffer device 405 may operate in a bandwidth concentrator approach. By employing quad channels 415 a–415 d on each of modules 450 a–450 d, bandwidth in each module may be concentrated from all quad channels 415 a–415 d on each module to corresponding point-to-point links 620 a–620 d. In this embodiment, throughput on each of point-to-point links 620 a–620 d is concentrated to four times the throughput achieved on each of quad channels 415 a–415 d. Here, each of channels 415 a–415 d transfers information between one or more respective memory devices on each channel and buffer device 405 simultaneously.

Any number of channels 415 a–415 d, for example; two channels 415 c and 415 d may transfer information simultaneously and the memory devices on the other two channels 415 a and 415 b remain in a ready or standby state until called upon to perform memory access operations. Different applications may have different processing throughput requirements. In addition, the throughput requirements of a particular application may dynamically change during processing. Typically, more power is consumed as throughput is increased as power consumption relates in proportion to operation frequency. The amount of throughput in a system may be implemented on a dynamic throughput requirement basis to save on power consumption. In this embodiment, memory system 700 may concentrate bandwidth as it is required while in operation. For example, memory system 700 may employ only one of channels 415 a–415 d and match throughput to the corresponding point-to-point link. As bandwidth requirements increase, memory system 700 may dynamically activate more of channels 415 a–415 d and increase the throughput on the point-to-point link along with the number of channels accordingly to meet the bandwidth requirements for a given operation.

With reference to FIG. 8A, a block diagram of a large capacity memory system according to an embodiment of the present invention is illustrated. Memory system 800 includes modules 470 a–470 p, coupled to controller 610 via repeaters 810 a–810 d, primary links 820 a–820 d, and repeater links 830 a–830 p. Primary links 820 a–820 d provide a point-to-point link between controller 610 and a respective repeater 810 a–810 d. In an embodiment of the present invention, each of repeaters 810 a–810 d decode packets transmitted from controller 610 which are then directed over one or more, or none of repeater links 830 a–d, depending on the type of access configured. Each repeater link 830 a–830 p may utilize a point-to-point link configuration. By incorporating, repeated links 830 a–830 p and repeaters 810 a–810 d, a larger number of modules may be accessed and a larger capacity memory system may be realized. Such a large capacity may be suited in a computer server system.

FIG. 8B illustrates another approach utilized to expand the memory capacity of a memory system in accordance to yet another embodiment. Here, a plurality of buffered modules 850 a–850 d are “daisy chained” via a plurality of point-to-point links 860 a–860 d to increase the overall memory capacity. Connection points of each point-to-point link are connected to two adjacent buffered modules. Each of buffered modules 850 a–850 d transceive signals between adjacent point-to-point links 860 a–860 d. Point-to-point link 860 a may be coupled to a controller or another buffered module. Additional point-to-point links may be coupled to a buffer device in a tree configuration approach. For example, three point-to-point links 870 a–870 c each having a single end connected to one buffer device 880 may be employed as shown in FIG. 8C.

Other point-to-point topologies include a “ring” in which a plurality of buffered modules are connected in a ring by a respective plurality of point-to-point links and a “star” where a plurality of memory modules are connected to a center memory module by a respective plurality of point-to-point links.

In various embodiment of the present invention, point-to-point links are unidirectional, bidirectional or a combination thereof. A unidirectional link transfers a signal in a single direction to or from a connection point. A bidirectional link transfers signals both to and from a connection point.

FIGS. 9A–24 illustrate a memory system, or portions of a memory system, for allowing an upgrade option for memory modules. In particular, FIGS. 9A–24 illustrate a memory system including a buffered memory module having at least one switch element that allows for memory module upgrades while reducing memory system delays in accessing information from the memory module upgrades.

FIG. 9A illustrates a memory system 910 including a single buffered memory module filled or populated in socket 904 and socket 905 having symmetrical switch elements 911 and 912 coupled to channels DQ0 and DQ1 and a memory system 920 including two buffered memory modules having symmetrical switch elements (SW) 911 and 912, respectively. Memory systems 910 and 920 include substrates 950 and 960, respectively, for positioning an interface, such as sockets 904 and 905 that may or may not be populated with buffered memory modules 900 and 901. The choice of populating sockets 904 and 905 with buffered memory modules 900 and 901 may be made at the time of manufacture, or after a memory system has been manufactured.

Memory modules 900 and 901 include a connector interface having contacts, such as pins A0 and A1 that are used to provide signals between memory modules 900 and 901 and sockets 904 and 905, respectively. Pins A0 and A1 represent one or more contacts or pins in embodiments of the present invention.

Data and/or control signals are transferred between buffered memory modules 900 and 901 by channels DQ0 and DQ1. In an alternate embodiment of the present invention, control and/or address signals are transferred on separate signal lines or channels. Channels DQ0 and DQ1 include one or more signal lines in embodiments of the present invention. In an embodiment of the present invention, channels DQ0 and DQ1 as well as other channels described herein, are unidirectional (information can travel in one direction) and/or bidirectional (information can travel in either direction). Channels DQ0 and DQ1 are coupled to a master device, such as memory controller 310 shown in FIG. 3B, in an embodiment of the present invention. Channel DQ0 is also coupled to pin A0 of memory module 900 by way of socket 904 and switch element 911. Channel DQ1 is also coupled to pin A1 of memory module 900 by way of socket 904 and switch element 912. Similarly, pin A1 of memory module 901 is coupled to channel DQ0 by way of switch element 911 and pin A0 of memory module 901 is coupled to channel DQ1 by way of switch element 912.

Switch elements 911 and 912 are coupled to channels DQ0 and DQ1 in an embodiment of the invention. Switch elements 911 and 912 permit a controlled impedance interconnect (impedance value of Z0) to couple to two other controlled impedance interconnects (of approximately the same impedance value Z0) with very little reflected energy from an incident signal waveform. This is performed by coupling a signal line in either channel DQ0 or DQ1 to either of two other signal lines in channel DQ0 or DQ1. Only one signal path of the two possible signal paths is selected at any time, so the impedance remains at approximately a Z0 value from a master device through a switch to a memory module, in particular a pin of a socket in an embodiment of the invention.

Switch elements 911 and 912 are positioned off of memory modules 900 and 901 but on substrates 950 and 960 in an embodiment of the invention. Switch elements 911 and/or 912 are positioned on memory modules 900 and/or 901 in an alternate embodiment of the invention. In a further embodiment of the invention, one or more switch elements is positioned in buffer device 930.

FIG. 9B illustrates an embodiment of a switch element. Switch element 980 is used as switch elements 911 and/or 912 shown in FIG. 9A in an embodiment of the invention. A switch element 980 includes two transistors 984 and 985. In an embodiment of the invention, transistor 984 is a negative metal oxide semiconductor (NMOS) device and transistor 985 is a positive metal oxide semiconductor (PMOS) device. In alternate embodiment of the invention, other types of semiconductor devices that provide a signal path are used.

Switch element 980 is coupled to signal lines 981, 982 and 983 in a channel, such as channel DQ0, in an embodiment. For example, switch element 980 is coupled to signal line 981 from a master device and coupled to signal lines 982 and 983 that are coupled to sockets 904 and 905, and in particular pins A0 and A1 of sockets 904 and 905, respectively. A control signal line for providing a select control signal (Sel) is also coupled to switch element 980. Transistor 985 includes source 985 a, gate 985 b and drain 985 c. Transistor 984 includes drain 984 a, gate 984 b and source 984 c. Source 985 a and drain 984 a are coupled to signal line 981. Drain 985 c is coupled to signal line 983. Source 984 c is coupled to signal line 982. Gates 984 b and 985 b are coupled to a control signal line and provide a connection or signal path between either signal lines 981 and 982 or signal lines 981 and 983 in response to a select control signal applied to gates 984 b and 985 b. In embodiments of the invention, a select control signal is provided by a master device, such as a memory controller, during initialization or during typical memory transactions. In an alternate embodiment of the invention, a select control signal is provide by programmable logic or state registers.

An advantage of using a switch element is that the number of ranks of memory modules in a memory system may be increased, while disrupting the integrity of the signal waveforms to a relatively small degree. A three branch topology, using two transistors described above, enables a signal behavior similar to a point-to-point topology.

A point-to-point topology has the benefit that the parasitic resistance, capacitance, and inductance elements in the controlled impedance path or signal line are minimized, thus maximizing the signaling rate (bandwidth). The signaling bandwidth is also kept high by the use of on-component termination elements, which provide a termination resistance that approximates the impedance value of the signal line.

Benefits of a point-to-point topology (coupling two components) may be largely realized in a switch topology described herein that couples three components. These benefits include relatively small parasitic resistance, capacitance, and inductance elements and on-component-termination (with a termination value of approximately Z0). These benefits allow the signaling rate (the signaling bandwidth) to be maximized.

Alternatively, a multi-drop topology couples more than two components together and has relatively large parasitic resistance, capacitance, and inductance elements due to large interconnect stubs coupled to the main interconnect structure and to the inability to provide on-component-termination which can completely absorb the incident signal (the termination element must be coupled to the main interconnect structure and not the interconnect stubs).

A disadvantage of using a switch element is that transistors must be selected or “turned on or off” on a transaction-by-transaction basis, so the settling time of the signal path should meet timing requirements in an embodiment of the invention. A master device, such as a controller, selects or generates a select control signal (Sel) to a switch element in an embodiment of the invention. At system initialization or during manufacture a select control signal may be generated and not thereafter during typical operation in an embodiment of the invention. FIGS. 9A–B illustrate a select control signal applied to a switch element 980 while other switch elements shown in other FIGURES do not explicitly show the select control signal. One of ordinary skill in the art understands that switch elements described and illustrated herein are provided with select control signals in embodiments of the invention.

In an embodiment of the present invention, memory modules 900 and 901, singly or in combination, include a buffer device 930 having termination components 935 a and 935 b coupled to pins A0 and A1, respectively, as indicated by the expanded view of buffered memory module 900 shown in FIG. 9A. In an embodiment of the invention, termination components 935 a and 935 b are resistors having impedance values of approximately Z0, respectively, in order to match the characteristic impedance of the signal lines of DQ0 and DQ1.

Buffer device 930 also includes mux/demux circuit 931 for selectively connecting pins A0 and A1 to memory devices 933, in particular memory cells, by way of internal channel 932. Mux/demux circuit 931 corresponds to multiplexer/demultiplexer circuit 597 shown in FIG. 5C in an embodiment of the present invention. Mux/demux circuit 931 performs transfers between timing slots on pins A0 and A1 and memory devices 933. In a write transfer embodiment of the present invention, mux/demux circuit 931 will receive information from pins A0 and A1, and will retransmit the information to the memory devices 933 where it is written into storage cells. In a read transfer embodiment of the present invention, mux/demux circuit 931 will receive information that is read from storage cells in memory devices 933, and will retransmit the information onto pins A0 and A1.

In an embodiment of the present invention, termination components 935 a–b and mux/demux circuit 931 are used in configurable width buffer device 391 shown in FIG. 5B.

In an embodiment of the invention, channels DQ0 and DQ1 shown in FIG. 9A are coupled to a master device, and in particular memory controller pins.

FIG. 10A illustrates the use of address space when using a single memory module 900 (signals are not provided to another memory module using switch elements 911 and 912—no connection (nc), but are provided from switch elements 911 and 912 to memory module 900). FIG. 10B illustrates the use of address space when using two memory modules 900 and 901. When memory module 900 is present, both channels DQ0 and DQ1 connect to memory module 900 through switch elements 911 and 912. When both memory modules 900 and 901 are present, channels DQ0 and DQ1 connect to memory modules 900 and 901 through switch elements 911 and 912 using the signals lines from switch elements 911 and 912 to pins A0 and A1, respectively, of memory modules 900 and 901. The address spaces of the memory modules 900 and 901 can be stacked, with an access to a particular address location involving only the memory devices of either memory module 900 or memory module 901. This permits different-sized modules (number of address locations) to be accommodated with no gaps or fragments in the address space. However, switch elements alternatively select between paths to memory module 900 and 901 on a transaction-by-transaction basis. The address space of memory modules 900 and 901 can be accessed in parallel as shown in FIG. 13B and described below.

FIGS. 11A–B illustrate a memory system 1100 having one to four memory modules 1101–1104 coupled to sockets 1105–1108 positioned on substrate 1150. Socket 1105 is coupled to channels DQ0–3 through switch elements 1120 a–d and 1121 a–d, respectively. Socket 1106 is coupled to channels DQ0–3 through switch elements 1121 a–d, respectively. Socket 1107 is coupled to channels DQ0–3 through switch elements 1120 a–d and 1122 a–d, respectively. Socket 1108 is coupled to channels DQ0–3 through switch elements 1120 a–d and 1122 a–d, respectively.

FIG. 11A illustrates a single memory module embodiment of the invention.

FIG. 11B illustrates a two memory module embodiment of the invention.

FIG. 11C illustrates a three memory module embodiment of the invention.

FIG. 11D illustrates a four memory module embodiment of the invention.

A cost of increasing the number of upgrade memory modules is that the DQ0–3 channels from a master device to a furthest memory module have two switch elements in series instead of one. A further cost is that more switch elements may be controlled, and more parasitic resistance, capacitance and inductance elements are present. A benefit of an embodiment illustrated in FIGS. 11A–D is that the number of upgrade sockets or slots has been increased to four (relative to two compared to FIG. 9A and relative to one for a point-to-point topology).

In an alternate embodiment of the present invention, each switch element couples a particular channel DQ0–3 to all four sockets 1105-1108 instead of two sockets as is shown in FIGS. 11A–D. In this embodiment, parasitic resistance, capacitance and inductance that are present in the selected signal path may be reduced. In this embodiment of the present invention, there are four switch elements coupled respectively to each channel DQ0–3. Each switch element is coupled to a signal line from a master device and four signal lines to respective sockets 1105–1108. Four transistors having respective sources (similar to transistor 985 shown in FIG. 9B) are coupled to a signal line in channel DQ0 from a master device and the four respective drains of the four transistors are coupled to the four respective signal lines in channel DQ0 that are respectively coupled to sockets 1105–1108. One or more control lines are coupled to the four respective gates of the four transistors in the switch element in an embodiment of the invention. Likewise, three other switch elements are coupled to channels DQ1–3. Accordingly, four switch elements with 5 connections to signal lines replace nine switch elements having three connections to signal lines, as shown in FIG. 11A–D, in an embodiment of the present invention.

In an embodiment of the present invention, an upgradeable memory system includes four or more memory modules without creating more parasitic loading than is provided by memory system 920 shown in FIG. 9A. FIG. 12 illustrates systems 1210 and 1220 including an asymmetrical switch element 912 that enables the reduced parasitic loading in memory systems having four or more memory modules. A difference between memory system 920 and systems 1210/1220 is that a switch element 912 is only coupled to channel DQ1 and A1 pins. There is no switch element provided for A0 pins of memory modules 900 and 901 in an asymmetrical embodiment of the invention. A0 pin of socket 905 is not connected (nc).

FIGS. 13A–B also show the address spaces for the one and two memory module cases with an asymmetrical switch element. The address space for memory module 900 shown in FIG. 13A is the same in memory module 900 shown in FIG. 9A. But in memory modules 900 and 901 shown in FIG. 13B, the address spaces of the two memory modules are parallel rather than stacked. This means that when an address is accessed, memory devices 933 on memory module 901 will be used for the transfer timing slots on the channel DQ1, and memory devices 933 on memory module 900 will by used for the transfer timing slots on channel DQ0.

In embodiments of the present invention, some (or all) of the storage locations or cells of memory devices 933 in memory module 900 shown in FIG. 13B must be accessible through either channel DQ0 or DQ1, depending upon the number of memory modules present. Mux/demux circuit 931 in buffer device 930 manages this accessing of storage locations/cells.

In an embodiment of the present invention, information from memory devices 933 that are accessed through a shorter path (without the switch element 912 and signal line segment 1310) must be delayed at some point so that the access paths are matched. This delay can be added at a master device, or can be added in the mux/demux circuit 931 of buffer device 930 in memory module 900 in embodiments of the present invention.

When using only memory module 900 shown in FIG. 13A, an access to a particular address in memory devices 933 causes a transfer between timing slots on channels DQ0 and DQ1 and storage locations in all memory devices 933 on memory module 900. Buffer device 930 handles the multiplexing and demultiplexing needed for performing this transfer.

When using two memory modules 900 and 901 (with equal-size storage capacity) shown in FIG. 13B, an access to a particular address causes a transfer between timing slots on channel DQ0 and storage locations in half of the memory devices 933 on memory module 900, and between timing slots on channel DQ1 and storage locations in half of the memory devices 933 on memory module 901. Buffer device 930, and in particular mux/demux 931, handles the multiplexing and demultiplexing needed for performing this transfer. Address and control signals from a master device will manage mux/demux 931 in an embodiment of the present invention.

In an embodiment of the present invention, the address space of memory modules 900 and 901 shown in FIG. 13B, respectively, is twice as large as the address space of memory module 900 shown in FIG. 13A, because only half of the memory devices 933 are being accessed in each transaction.

An advantage of accessing parallel address space as shown in FIG. 13B is that the selected path through a switch element may be chosen at system initialization and not changed during typical operation. This reduces the timing constraints upon a switch element and potentially allows system performance to be higher (if a signal path to a memory module during typical operation is selected by a switch element, it may be necessary to allow a settling time before the selected signal path is used for a transaction).

When memory modules 1400 and 1401, as shown in FIG. 14B, do not have equal storage capacities, then the management of the address space becomes more complicated. For example, when the upgrade memory module 1401 has half the storage capacity of memory module 1400, the address space increases in size by a factor of 1.5. Accesses to address locations in the first ⅔ of the address space cause transfers from both memory modules 1400 and 1401.

When the upper ⅓ of the address space is accessed, only memory module 1400 is accessed in an embodiment of the present invention. There are at least two ways in which access can be performed in embodiments of the present invention.

First, the timing slots on channels DQ0 and DQ1 are both used by memory module 1400 in an embodiment of the present invention. A master device uses address and control signals to cause memory module 1400 to perform double-width transfers in this region of the address space as illustrated by FIG. 15A. A signal path selected by switch element 912 in FIG. 15A is changed between memory module 1400 and 1401, depending upon the address selected.

Second, the timing slot on channel DQ0 is filled from memory module 1400, and the timing slot on channel DQ1 is not used. A master device knows that access to this part of the address space will be at half the nominal bandwidth, and that the master device performs the appropriate multiplexing and demultiplexing to the master device's internal signal paths as shown by FIG. 15B. In this embodiment of the invention, switch element 912 shown in FIG. 15B does not cause a change in a signal path: memory module 1400 is always selected.

The embodiment illustrated in FIG. 15A is preferred to the embodiment illustrated in FIG. 15B, since it provides full bandwidth access to the entire address space. Further, the buffer device 930 includes mux/demux 931 for handling different access sizes in the lower ⅔ and the upper ⅓ of the address space and switch element 912 selects between two signal paths to memory modules 1400 and 1401 on a transaction-by-transaction basis.

FIGS. 16A–D illustrate memory systems 1600, 1630, 1650 and 1670 including asymmetric switch elements for memory module upgrades. FIGS. 16A–D illustrate how more memory modules can be added to a two memory module system without additional switch elements inserted serially on each channel DQ0–3. In an embodiment of the invention, a master device such as a memory controller is coupled to channels DQ0–3 for transferring information to and from memory modules 1601–1604. Channels DQ0–3 are coupled to switch elements 1641 a–d, respectively, and sockets 1605–1608 (through switch elements 1641 a–d) in an embodiment of the invention. Channels DQ0–3 are coupled to socket 1605 by way of four sets of pins (through switch elements 1641 a–d, respectively). Channels DQ0–1 are coupled to socket 1606 by way of two sets of pins (through switch elements 1641 a–b, respectively). Channel DQ2 is coupled to socket 1607 by way of one set of pins (through switch element 1641 c). Channel DQ3 is coupled to socket 1608 by way of one set of pins (through switch element 1641 d).

FIG. 16A shows a one-module case 1600 where memory module 1601 is coupled to socket 1605 and sockets 1606–08, respectively. In a one-module case, an access causes a transfer between a timing slot on all four channels DQ0, DQ1, DQ2, and DQ3 and storage locations in memory module 1601 (the top socket 1605).

FIG. 16B shows a two-module case 1630 where memory modules 1601 and 1602 are coupled to sockets 1605 and 1606, respectively. An access causes a transfer between a timing slot on two channels DQ2 and DQ3 and storage locations in memory module 1601 (the top socket 1606) and between a timing slot on two channels DQ0 and DQ1 and storage locations in memory module 1602 (the second socket 1606 from the top).

FIG. 16C shows a three-module case 1650 where memory modules 1601, 1602 and 1603 are coupled to sockets 1605, 1606 and 1607, respectively. An access causes a transfer between a timing slot on two channels DQ0 and DQ3 and storage locations in memory module 1601 (the top socket 1605), between a timing slot on channel DQ1 and storage locations in memory module 1602 (the second socket 1606 from the top), and between a timing slot on channel DQ2 and storage locations in memory module 1603 (the third socket 1607 from the top).

FIG. 16D shows a four-module case 1670 where memory modules 1601, 1602, 1603 and 1604 are coupled to sockets 1605, 1606, 1607 and 1608, respectively. An access causes a transfer between a timing slot on channel DQ0 and storage locations in memory module 1601 (the top socket 1605), between a timing slot on channel DQ1 and storage locations in memory module 1602 (the second socket 1606 from the top), between a timing slot on channel DQ2 and storage locations in memory module 1603 (the third socket 1607 from the top), and between a timing slot on channel DQ3 and storage locations in memory module 1604 (the fourth socket 1608 from the top).

Signals on channels DQ0–3, shown in FIGS. 16A–D, encounter the same number of switch elements (one) as in the two memory module embodiments illustrated by FIGS. 9A and 12. However, a four memory module embodiment illustrated by FIGS. 16A–D will provide greater capacity and more upgrade opportunities. In alternate embodiments of the present invention, more than four modules are used in a memory system.

In various embodiments of the present invention, each memory system shown in FIGS. 16A–D will have many combinations of memory module storage capacities; i.e. all the memory modules have the same storage capacity, some memory modules have twice the storage capacity of other memory modules, some memory modules have four times the storage capacity of other memory modules, and so forth. In a preferred embodiment, a memory module with the largest storage capacity will be inserted into a higher socket position; i.e. toward the top of FIG. 16-D. This will enable higher storage capacity memory modules the opportunity to use a larger number of channels.

For some memory module storage capacity combinations, it will be possible to provide uniform access bandwidth across the entire combined address space. For example, for a three-module case shown in FIG. 16C, if the top memory module 1601 has twice the storage capacity of the other two memory modules 1602 and 1603, then uniform access bandwidth is available at any address location. This is because memory module 1601 has two timing slots (channels DQ0 and DQ3) for every single timing slot of memory module 1602 (channel DQ1) and memory module 1603 (channel DQ2).

For some of the storage capacity combinations, it will not be possible to provide uniform access bandwidth across the entire combined address space using the fixed channels shown in FIGS. 16A–D in an embodiment of the present invention. For example, for the three-module case 1650 shown in FIG. 16C, if the top memory module 1601 has four times the storage capacity of the other two memory modules 1602 and 1603, then there will be a bandwidth mismatch when the upper half of memory module 1601 is accessed.

There are at least two ways in which access can be performed in embodiments of the present invention.

First, the timing slots on channels DQ0, DQ1, DQ2 and DQ3 are all filled from memory module 1601 when the address locations in the upper half of memory module 1601 are accessed. This requires that a master device use the address and control signals to cause memory module 1601 to perform double-width transfers in this region of the address space, similar to the two-module case shown in FIG. 15A. A signal path selected by switch element 912 in FIG. 15A is changed between memory module 1400 and 1401, depending upon the address selected on a transaction-by-transaction basis. This accessing is preferable since it provides full bandwidth access to the entire address space and does not require a buffer device to include multiplexing logic for handling the different access sizes in the lower ⅔ and the upper ⅓ of the address space.

Second, the timing slots on channels DQ0 and DQ3 are filled from memory module 1601 and the timing slots on channel DQ1 and DQ2 are not used. In this embodiment of the present invention, a master device knows that access to this part of the address space will be at half the nominal bandwidth, and that it performs the appropriate multiplexing and demultiplexing to its internal signal paths. This embodiment is similar to the two-module case shown in FIG. 15B.

FIGS. 17–24 illustrate accessing storage cells by external channels such as channels DQ0 and DQ1 that include switch elements, in particular accessing storage cells by signal lines data 0 and data 1 using a buffer device, such as buffer device 930, in a buffered memory module in embodiments of the present invention.

FIG. 17 illustrates operation modes of a memory module 1700 that couples external signal lines data 0 and data 1 to storage cells Sa and Sb in memory device 1702 by buffer device 1701. In embodiments of the present invention, memory module 1700 corresponds to memory module 900 or 1400 described above. In embodiments of the present invention, signal lines data 0 and data 1 are included in a single external channel, such as channel DQ0, or respective external channels. In still a further embodiment of the present invention, an external signal line is positioned on a memory module substrate between a buffer device and a memory module contact.

Memory module 1700 includes memory device 1702 including storage cells Sa and Sb that represent a memory device in memory devices 933 as described herein in an embodiment of the present invention. Likewise, buffer device 1701 represents a buffer device as described herein, such as buffer device 930, in embodiments of the present invention.

In a first mode of operation at a first time, buffer device 1701 couples signal line data 0 to storage cell Sa and signal line data 1 to storage cell Sb.

In a second mode of operation at a second time, buffer device 1701 couples signal line data 0 to storage cell Sa and signal line data 1 does not have access to memory module 1700.

In a third mode of operation at a third time, buffer device 1701 couples signal line data 0 to storage cell Sb and signal line data 1 does not have access to memory module 1700.

FIG. 18 illustrates operation modes of a memory module 1800 that couples external signal lines data 0 and data 1 to storage cells Sa and Sb. In embodiments of the present invention, external signal lines data 0 and data 1 are included in a single external channel, such as channel DQ0, or respective external channels. In still a further embodiment of the present invention, an external signal line is positioned on a memory module substrate between a buffer device and a memory module contact. Memory module 1800 includes a memory device 1802 including storage cell Sa and a memory device 1803 including storage cell Sb that represent respective memory devices in memory devices 933 as described herein in an embodiment of the present invention.

Memory module 1800 operates similar to memory module 1700, as described above, except that two memory devices 1802 and 1803 having respective storage cells Sa and Sb are accessed instead of storage cells from a single memory device.

FIG. 19 illustrates operation modes of a memory module 1900 that couples external signal lines data 0 and data 1 to storage cells Sa, Sb and Sc. In embodiments of the present invention, signal lines data 0 and data 1 are included in a single external channel, such as channel DQ0, or respective external channels. In still a further embodiment of the present invention, an external signal line is positioned on a memory module substrate between a buffer device and a memory module contact.

Memory module 1900 includes memory device 1902 including storage cells Sa, Sb and Sc that represent a memory device in memory devices 933 as described herein in an embodiment of the present invention. Likewise, buffer device 1701 represents a buffer device, such as buffer device 930, as described herein in embodiments of the present invention.

In a first mode of operation at a first time, buffer device 1701 couples signal line data 0 to storage cell Sa and signal line data 1 to storage cell Sb.

In a second mode of operation at a second time, buffer device 1701 couples signal line data 0 to storage cell Sc and signal line data 1 does not have access to memory module 1900.

FIG. 20 illustrates operation modes of a memory module 2000 that couples external signal lines data 0 and data 1 to storage cells Sa, Sb, Sc, Sd. In embodiments of the present invention, signal lines data 0 and data 1 are included in a single external channel, such as channel DQ0, or respective external channels. In still a further embodiment of the present invention, an external signal line is positioned on a memory module substrate between a buffer device and a memory module contact.

Memory module 2000 includes a first memory device 2002 including storage cells Sa and Sc; and a second memory device 2003 including storage cells Sb and Sd that represent respective memory devices in memory devices 933 as described herein in an embodiment of the present invention. Likewise, buffer device 1701 represents a buffer device, such as buffer device 930, as described herein in embodiments of the present invention.

In a first mode of operation at a first time, buffer device 1701 couples signal line data 0 to storage cell Sa and signal line data 1 to storage cell Sc.

In a second mode of operation at a second time, buffer device 1701 couples signal line data 0 to storage cell Sc and data line 1 has no access to memory module 2000.

In a third mode of operation at a third time, buffer device 1701 couples signal line data 0 to storage cell Sd and signal line data 1 has no access to memory module 2000.

FIG. 21 illustrates operation modes of memory modules 2100 and 2101 that couple external signal lines data 0 and data 1 to storage cells Sa, Sb Sc and Sd in memory devices 2102 and 2103 by buffer device 1701. In embodiments of the present invention, memory modules 2100 and 2101 correspond to memory module 900 described above. In embodiments of the present invention, signal lines data 0 and data 1 are included in a single external channel, such as channel DQ0, or respective external channels. In still a further embodiment of the present invention, an external signal line is positioned on a memory module substrate between a buffer device and a memory module contact.

Memory module 2100 includes memory device 2102 including storage cells Sa and Sb that represent a memory device in memory devices 933 as described herein in an embodiment of the present invention. Likewise, buffer device 1701 represents a buffer device as described herein, such as buffer device 930, in embodiments of the present invention.

Memory module 2101 includes memory device 2103 including storage cells Sc and Sd that represent a memory device in memory devices 933 as described herein in an embodiment of the present invention.

Switch elements 2111 and 2110 couple signal lines data 0 and data 1 to memory modules 2100 and 2101 in various operation modes. In an embodiment of the invention, switch element 2111 is not used and signal line data 0 is directly coupled to a memory module(s). In embodiments of the invention, switch elements 2111 and/or 2110 correspond to switch element 980 shown in FIG. 9B and described above.

In a first mode of operation at a first time, buffer device 1701 of memory module 2100 couples signal line data 0 to storage cell Sa by way of switch element 2111 and signal line data 1 to storage cell Sb by way of switch element 2110.

In a second mode of operation at a second time, buffer device 1701 of memory module 2100 couples signal line data 0 to storage cell Sa by way of switch element 2111 and signal line data 1 does not have access to memory module 2101. Buffer device 1701 of memory module 2101 couples signal line data 1 by way of switch element 2110 to storage cell Sc and signal line data 0 does not have access to memory module 2101.

In a second mode of operation at a third time, buffer device 1701 of memory module 2100 couples signal line data 0 to storage cell Sb by way of switch element 2111 and signal line data 1 does not have access to memory module 2101. Buffer device 1701 of memory module 2101 couples signal line data 1 by way of switch element 2110 to storage cell Sd and signal line data 0 does not have access to memory module 2101.

FIG. 22 illustrates operation modes of memory modules 2200 and 2201 that couple external signal lines data 0 and data 1 to storage cells Sa, Sb, Sc and Sd in memory devices 2202, 2203, 2204 and 2205 by buffer device 1701. In embodiments of the present invention, memory modules 2200 and 2201 correspond to memory module 900 described above. In embodiments of the present invention, signal lines data 0 and data 1 are included in a single external channel, such as channel DQ0, or respective external channels. In still a further embodiment of the present invention, an external signal line is positioned on a memory module substrate between a buffer device and a memory module contact.

Memory modules 2200 and 2201 operate similar to memory modules 2100 and 2101, as described above, except that particular memory devices 2202, 2203 and 2204 having storage cells Sa, Sb, Sc and Sd are accessed instead of particular storage cells from a single memory device.

FIG. 23 illustrates operation modes of memory modules 2300 and 2301 that couple external signal lines data 0 and data 1 to storage cells Sa, Sb Sc and Sd in memory devices 2302 and 2304 by buffer device 1701. In embodiments of the present invention, memory modules 2300 and 2301 correspond to memory module 900 described above. In embodiments of the present invention, signal lines data 0 and data 1 are included in a single external channel, such as channel DQ0, or respective external channels. In still a further embodiment of the present invention, an external signal line is positioned on a memory module substrate between a buffer device and a memory module contact.

Memory module 2300 includes memory device 2302 including storage cells Sa, Sb and Sc that represent a memory device in memory devices 933 as described herein in an embodiment of the present invention. Likewise, buffer device 1701 represents a buffer device as described herein, such as buffer device 930, in embodiments of the present invention.

Memory module 2301 includes memory device 2304 including storage cell Sd that represents a memory device in memory devices 933 as described herein in an embodiment of the present invention.

In a first mode of operation at a first time, buffer device 1701 of memory module 2300 couples signal line data 0 to storage cell Sa by way of switch element 2111 and signal line data 1 to storage cell Sb by way of switch element 2110. Signal lines data 0 and data 1 do not have access to memory module 2301.

In a second mode of operation at a second time, buffer device 1701 of memory module 2300 couples signal line data 0 to storage cell Sc by way of switch element 2111 and signal line data 1 does not have access to memory module 2300. Buffer device 1701 of memory module 2301 couples signal line data 1 by way of switch element 2110 to storage cell Sd and signal line data 0 does not have access to memory module 2301.

FIG. 24 illustrates operation modes of memory modules 2400 and 2401 that couple external signal lines data 0 and data 1 to storage cells Sa, Sb, Sc and Sd in memory devices 2402, 2403 and 2404 by buffer device 1701. In embodiments of the present invention, memory modules 2400 and 2401 correspond to memory module 900 described above. In embodiments of the present invention, signal lines data 0 and data 1 are included in a single external channel, such as channel DQ0, or respective external channels. In still a further embodiment of the present invention, an external signal line is positioned on a memory module substrate between a buffer device and a memory module contact.

Memory modules 2400 and 2401 operate similar to memory modules 2300 and 2301, as described above, except that storage cells Sa and Sc are in memory device 2403 and storage cell Sb is in memory device 2402 in memory module 2400.

While this invention has been described in conjunction with what is presently considered the most practical embodiments, the invention is not limited to the disclosed embodiments. In the contrary, the embodiments disclosed cover various modifications that are within the scope of the invention as set forth in the following claims. 

1. A memory subsystem, comprising: a first memory module including, a first integrated circuit having memory including a first storage cell and a second storage cell, a first buffer device coupled to the first integrated circuit; a second memory module including, a second integrated circuit having memory including a third storage cell and a fourth storage cell, a second buffer device coupled to the second integrated circuit; an interconnect including a first signal line and a second signal line; and a first switch element coupling the second signal line to the first memory module, the first switch element having an input to receive a control signal, wherein the first switch element determines which of the first and second buffer devices is accessed via the second signal line based on the control signal, wherein the memory subsystem is operable in a first mode of operation and a second mode of operation, wherein: during the first mode of operation, the first storage cell is accessible from the first signal line through the first buffer device and the second storage cell is accessible from the second signal line through the first switch element and the first buffer device, and during the second mode of operation, the first storage cell is accessible from the first signal line through the first buffer device and the third storage cell is accessible from the second signal line through the first switch element and the second buffer device.
 2. The memory subsystem of claim 1, wherein: during the second mode of operation, at a second time, the second storage cell is accessible from the first signal line through the first buffer device and the third storage cell is accessible from the second signal line through the switch element and the second buffer device.
 3. The memory subsystem of claim 1, further comprising: a second switch element coupled to the first signal line and the first memory module.
 4. The memory subsystem of claim 1, wherein the first switch element includes a first and second transistor.
 5. The memory subsystem of claim 4, wherein the first transistor is an NMOS transistor and the second transistor includes a PMOS transistor.
 6. The memory subsystem of claim 5, wherein the NMOS transistor has a gate and the PMOS transistor has a gate coupled to the NMOS gate.
 7. A memory subsystem, comprising: a first memory module including, a first integrated circuit having memory including a first storage cell, a second integrated circuit having memory including a second storage cell, a first buffer device coupled to the first and second integrated circuits; a second memory module including, a third integrated circuit having memory including a third storage cell, a fourth integrated circuit having memory including a fourth storage cell, a second buffer device coupled to the third and fourth integrated circuits; an interconnect including a first signal line and a second signal line; and a first switch element coupling the second signal line to the first memory module, the first switch element having an input to receive a control signal, wherein the first switch element determines which of the first and second buffer devices is accessed via the second signal line based on the control signal, wherein the memory subsystem is operable in a first mode of operation and a second mode of operation, wherein: during the first mode of operation, the first storage cell is accessible from the first signal line through the first buffer device and the second storage cell is accessible from the second signal line through the first switch element and the first buffer device, and during the second mode of operation, the first storage cell is accessible from the first signal line through the first buffer device and the third storage cell is accessible from the second signal line through the first switch element and the second buffer device.
 8. The memory subsystem of claim 7, wherein: during the second mode of operation, at a second time the second storage cell is accessible from the first signal line through the first buffer and the fourth storage cell is accessible from the second signal line through the first switch element and the second buffer device.
 9. The memory subsystem of claim 7, further comprising: a second switch element coupled to the first signal line and the first memory module.
 10. The memory subsystem of claim 7, wherein the first switch element includes a first and second transistor.
 11. The memory subsystem of claim 10, wherein the first transistor is an NMOS transistor and the second transistor includes a PMOS transistor.
 12. The memory subsystem of claim 11, wherein the NMOS transistor has a gate and the PMOS transistor has a gate coupled to the NMOS gate.
 13. A memory subsystem, comprising: a first memory module including, a first integrated circuit having memory including a first, second and third storage cell, a first buffer device coupled to the first integrated circuit; a second memory module including, a second integrated circuit having memory including a fourth storage cell, a second buffer device coupled to the second integrated circuit; an interconnect including a first signal line and a second signal line; and a first switch element coupling the second signal line to the first memory module and the second memory module, the first switch element having an input to receive a control signal, wherein the first switch element determines which of the first and second buffer devices is accessed via the second signal line based on the control signal, wherein the memory subsystem is operable in a first mode of operation and a second mode of operation, wherein: during the first mode of operation at a first time, the first storage cell is accessible from the first signal line through the first buffer device and the second storage cell is accessible from the second signal line through the first switch element and the first buffer device, and during the second mode of operation at a second time, the third storage cell is accessible from the first signal line through the first buffer device and the fourth storage cell is accessible from the second signal line through the first switch element and the second buffer device.
 14. The memory subsystem of claim 13, further comprising: a second switch element coupled to the first signal line and the first memory module.
 15. The memory subsystem of claim 13, wherein the first switch element includes a first and second transistor.
 16. The memory subsystem of claim 15, wherein the first transistor is an NMOS transistor and the second transistor includes a PMOS transistor.
 17. The memory subsystem of claim 16, wherein the NMOS transistor has a gate and the PMOS transistor has a gate coupled to the NMOS gate.
 18. A memory subsystem, comprising: a first memory module including, a first integrated circuit having memory including a first and a third storage cell, a second integrated circuit having memory including a second storage cell, a first buffer device coupled to the first and second integrated circuits; a second memory module including, a third integrated circuit having memory including a fourth storage cell, a second buffer device coupled to the third integrated circuit; an interconnect including a first signal line and a second signal line; and a first switch element coupling the second signal line to the first memory module, the first switch element having an input to receive a control signal, wherein the first switch element determines which of the first and second buffer devices is accessed via the second signal line based on the control signal, wherein the memory subsystem is operable in a first mode of operation and a second mode of operation, wherein: during the first mode of operation at a first time, the first storage cell is accessible from the first signal line through the first buffer device and the second storage cell is accessible from the second signal line through the first switch element and the first buffer device, and during the second mode of operation at a second time, the third storage cell is accessible from the first signal line through the first buffer device and the fourth storage cell is accessible from the second signal line through the first switch element and the second buffer device.
 19. The memory subsystem of claim 18, further comprising: a second switch element coupled to the first signal line and the first memory module.
 20. The memory subsystem of claim 19, wherein the first switch element includes a first and second transistor.
 21. The memory subsystem of claim 20, wherein the first transistor is an NMOS transistor and the second transistor includes a PMOS transistor.
 22. The memory subsystem of claim 21, wherein the NMOS transistor has a gate and the PMOS transistor has a gate coupled to the NMOS gate.
 23. A system, comprising: a controller coupled to a first, second, third and fourth channel; a buffer device; a first and second switch element coupled to the first channel; a third and fourth switch element coupled to the second channel; a fifth and sixth switch element coupled to the third channel; a seventh and eighth switch element coupled to the fourth channel; a ninth switch element coupled to the first switch element; a tenth switch element coupled to the third switch element; an eleventh switch element coupled to the fifth switch element; a twelfth switch element coupled to the seventh switch element; a first memory module coupled to the second, fourth, sixth and eighth switch elements; a second memory module coupled to the second, fourth, sixth and eighth switch elements; a third memory module coupled to the ninth, tenth, eleventh and twelfth switch elements; and a fourth memory module coupled to the ninth, tenth, eleventh and twelfth switch elements.
 24. An apparatus, comprising: a first memory module having a first buffer device; a second memory module having a second buffer device; and means for switching information between the first and second memory modules in response to a select signal, the means for switching having an input to receive a control signal, wherein the means for switching determines which of the first and second buffer devices is accessed based on the control signal, wherein the means for switching information is operable in a first mode of operation and a second mode of operation, wherein: during the first mode of operation, a first storage cell in the first memory module is accessible from a first signal line through the first buffer device and a second storage cell in the first memory module is accessible from a second signal line through the first buffer device, and during the second mode of operation, the first storage cell is accessible from the first signal line through the first buffer device and a third storage cell in the second memory module is accessible from the second signal line through a second buffer device.
 25. A memory subsystem, comprising: a first memory module including, a first integrated circuit having memory including a first storage cell and a second storage cell, a first buffer device coupled to the first integrated circuit; a second memory module including, a second integrated circuit having memory including a third storage cell and a fourth storage cell, a second buffer device coupled to the second integrated circuit; an interconnect including a first signal line and a second signal line; and a first switch element coupling the second signal line to the first memory module, the first switch element having an input to receive a control signal, wherein the first switch element determines which of the first and second buffer devices is accessed via the second signal line based on the control signal, wherein the control signal is provided during initialization, wherein the memory subsystem is operable in a first mode of operation and a second mode of operation, wherein: during the first mode of operation, the first storage cell is accessible from the first signal line through the first buffer device and the second storage cell is accessible from the second signal line through the first switch element and the first buffer device, and during the second mode of operation, the first storage cell is accessible from the first signal line through the first buffer device and the third storage cell is accessible from the second signal line through the first switch element and the second buffer device.
 26. The memory subsystem of claim 25, wherein the control signal is provided by a register.
 27. The memory subsystem of claim 25, wherein the control signal is provided by a memory controller.
 28. The memory subsystem of claim 27, wherein the control signal is further provided by a memory controller during a memory transaction. 